Vapor cells having one or more optical windows bonded to a dielectric body

ABSTRACT

In a general aspect, a vapor cell is presented that includes a dielectric body. The dielectric body has a surface that defines an opening to a cavity in the dielectric body. The vapor cell also includes a vapor or a source of the vapor in the cavity of the dielectric body. An optical window covers the opening of the cavity and has a surface bonded to the surface of the dielectric body to form a seal around the opening. The seal includes metal-oxygen bonds formed by reacting a first plurality of hydroxyl ligands on the surface of the dielectric body with a second plurality of hydroxyl ligands on the surface of the optical window.

BACKGROUND

The following description relates to vapor cells and their methods ofmanufacture.

Vapor cells are manufactured by sealing a vapor or gas within anenclosed volume. To introduce the vapor or gas into the enclosed volume,a vapor cell may include a delivery tube formed of glass, called a stem,that is fused-off when sufficient vapor or gas has entered the enclosedvolume. The fused-off delivery tube, however, projects from the vaporcell, making the vapor cell fragile, awkward to package, and may alsoperturb electric fields measured by the vapor cell. Vapor cells are alsocommonly manufactured using an anodic bonding process. The anodicbonding process requires high temperatures and voltages and may outgasvolatile species during bond formation. The high temperatures preventanti-relaxation coatings from being applied to the vapor cell, and thevolatile species may contaminate the vapor or gas in the vapor cell.Vapor cells and methods of manufacture are desired that avoid ormitigate these shortcomings.

DESCRIPTION OF DRAWINGS

FIG. 1A is an exploded view, in perspective, of an example vapor cellhaving a dielectric body and an optical window;

FIG. 1B is a perspective view of the example vapor cell of FIG. 1A, butin which the optical window is contact-bonded to the dielectric body;

FIG. 2A is an exploded view, in perspective, of an example vapor cellhaving two optical windows;

FIG. 2B is a perspective view of the example vapor cell of FIG. 2A, butin which both optical windows are bonded to a dielectric body of theexample vapor cell;

FIG. 3 is an example vapor cell, shown from perspective, top, and sideviews, that has a dielectric body formed of silicon and two opticalwindows formed of glass;

FIG. 4 is an example vapor cell, shown from perspective, top, and sideviews, that has a dielectric body formed of glass and two opticalwindows formed of glass;

FIG. 5A is a perspective view of an example vapor cell that includes acircular dielectric body and a cavity defined by a square outer wall anda circular inner wall of the circular dielectric body;

FIG. 5B is a perspective view of an example vapor cell that includes asquare dielectric body and a cavity is defined by circular outer andinner walls of the square dielectric body; and

FIG. 5C is a perspective view of an example vapor cell that includes acircular dielectric body and a cavity defined by circular outer andinner walls of the circular dielectric body.

DETAILED DESCRIPTION

Many applications in atom-based sensing require stemless, small vaporcells with high purity gas samples inside them. These types of vaporcells are challenging to manufacture in large numbers, especially whenthe vapor cells require well-controlled pure samples of atoms ormolecules. Currently, the best vapor cells for such applications aremade using anodic bonding, which requires high temperatures and highvoltages. However, during the anodic bonding process, outgassing oftentakes place in the vapor cell, introducing volatile contaminants intothe vapor inside. Such contaminants may reduce the performance of vaporcells during atom-based sensing applications. Moreover, the hightemperatures and voltages associated with anodic bonding also preventanti-relaxation coatings from being applied to the vapor cells.

It is also undesirable to have a stem on vapor cells for manyapplications such as magnetometry, time sensing, and atom-based electricfield sensing. The stem makes the vapor cell fragile, awkward topackage, and perturbs the electromagnetic field to be measured. Foratom-based sensing of electromagnetic fields, it is important to havethe size of the vapor cell much smaller than a wavelength of theelectromagnetic field to be detected. The presence of a stem hinders theminiaturization of vapor cells to dimensions smaller than wavelengths ofelectromagnetic fields to be detected. Furthermore, it is desirable forthe vapor cells to be manufactured so that they allow the transmissionof light into the cavity containing the atoms and/or molecules.

Small vapor cells without stems that can hold pure samples of atoms ormolecules are important to atom and molecule-based sensing. Applicationssuch as atomic clocks and sensors based on vapor cells can allow roomtemperature operation of useful metrological devices that areself-calibrated and whose performance can surpass conventional devices.As the world becomes more standardized and connected these types ofinstruments become increasingly important as the demands placed on theprecision and accuracy of electromagnetic field sensing and timinggrows. For example, field calibration of radar systems throughover-the-air testing requires standard measurements that usually takeplace in large test facilities. Clocks and accelerometers that can meetthe demanding applications of GPS for GPS-denied areas as well as handleGPS dropouts can benefit from such vapor cells. Current coherentpopulation trapping (CPT) atomic clocks operate with large amounts ofbuffer gas that make it possible to use anodic bonding to seal the vaporcells used in these applications. However, pure vapor samples of alkaliatoms can enable other kinds of devices including different types ofclocks. For example, vapor cell magnetometers that requirespin-preserving, anti-relaxation coatings are another application ofthis vapor cell preparation method.

In some aspects of what is described here, vapors cells for atom-basedsensing are presented, including their methods of manufacture. In manyvariations, the vapor cells are small (e.g., less than 1 mm³) andstemless. However, larger vapor cells with variable geometry are alsopossible. The vapor cells may be sufficiently small, when compared to awavelength of electromagnetic fields measured by the vapor cells, thattheir scattering cross-section is reduced below a geometriccross-section. The lack of a stem may also reduce the scatteringcross-section as well as reducing a distortion of the measuredelectromagnetic fields. The methods of manufacture are capable ofproducing such vapor cells. The methods may be based on fabricatingarrays of stemless vapor cells on large substrates and then cutting outthe vapor cells with either a dicing saw or laser. However, the methodsmay also be based on individual chips that correspond to a singlestemless vapor cell.

In the methods, a frame is prepared that can be pumped out to lowpressure (e.g., less than 10⁻⁹ Torr). The frame may be fabricated bymaking cavities or holes in a substrate. If holes are made, an opticalwindow or backing can be bonded with a strong, high temperature bondsuch as an anodic bond or glass frit bond. Outgassing in such operationsdoes not affect the vapor cell because the openings are not sealed andthus contaminant gases can escape. To seal a high purity gas in thevapor cell, the methods produce a contact bond in a backgroundenvironment of the pure gas. The contact bond can also be formed in purevacuum if a filling operation is performed, e.g., dotting a cavity orholes in the frame with an alkali metal, inserting a vaporizable sourceof alkali atoms in the cavity or holes, and so forth.

The contact bond to seal the vapor cell works by chemically activatingthe surfaces of the frame and the optical window using a plasma. Suchactivation may be supplemented with a wash to create hydroxyl bonds(e.g., bonds to OH chemical groups), which eventually form metal-oxygenbonds between the frame and window/cover. For example, if the frame isformed of silicon oxide (e.g., vitreous silica, single-crystal quartz,etc.) and the optical window is a glass that includes silicon oxide, themetal-oxygen bonds may include siloxane bonds (i.e., Si—O—Si). Siloxanecontact bonding works well with frames and optical windows that includesilicon oxide. However, other materials may be possible if coated with alayer of silicon oxide. For example, the front surface of a siliconframe may be coated with a layer of silicon oxide (e.g., SiO₂, SiO_(x),etc.) to facilitate a contact bond between the optical window and theframe. After contact bonding, the vapor cell can be easily fiber coupledby attaching a fiber to the window(s) directly or attaching a GRIN lensattached to a fiber to the vapor cell. Such coupling may allow the vaporcell to function as a sensor for sensing of electromagnetic fields.

Now referring to FIG. 1A, an exploded view is presented, in perspective,of an example vapor cell 100 having a dielectric body 102 and an opticalwindow 104. FIG. 1B presents the example vapor cell 100 of FIG. 1A, butin which the optical window 104 is contact-bonded to the dielectric body102. The example vapor cell 100 is stemless, and in many variations, isless than 1 mm³ in size. The dielectric body 102 may be a substratedefined by opposing planar surfaces, as shown in FIGS. 1A-1B. However,other configurations are possible for the dielectric body 102. Moreover,although FIGS. 1A-1B depict the dielectric body 102 as being square,other shapes are possible. The optical window 104 may also be asubstrate defined by opposing planar surfaces. However, otherconfigurations are possible for the optical window 104. In general, theoptical window 104 includes one surface adapted to mate (or bond)against a surface of the dielectric body 102, thereby allowing a seal toform (e.g., via a contact bond).

The dielectric body 102 may be formed of a material transparent toelectric fields (or electromagnetic radiation) measured by the vaporcell 100. The material may be an insulating material having a highresistivity, e.g., ρ>10⁸ Ω·cm, and may also correspond to a singlecrystal, a polycrystalline ceramic, or an amorphous glass. For example,the dielectric body 102 may be formed of silicon. In another example,the dielectric body 102 may be formed of a glass that includes siliconoxide (e.g., SiO₂, SiO_(x), etc.), such as vitreous silica, aborosilicate glass, or an aluminosilicate glass. In some instances, thematerial of the dielectric body 102 is an oxide material such asmagnesium oxide (e.g., MgO), aluminum oxide (e.g., Al₂O₃), silicondioxide (e.g., SiO₂), titanium dioxide (e.g., TiO₂), zirconium dioxide,(e.g., ZrO₂), yttrium oxide (e.g., Y₂O₃), lanthanum oxide (e.g., La₂O₃),and so forth. The oxide material may be non-stoichiometric (e.g.,SiO_(x)), and may also be a combination of one or more binary oxides(e.g., Y:ZrO₂, LaAlO₃, etc.). In other instances, the material of thedielectric body 102 is a non-oxide material such as silicon (Si),diamond (C), gallium nitride (GaN), calcium fluoride (CaF), and soforth. In these instances, an adhesion layer may be disposed on thedielectric body 102 to define the surface 106 of the dielectric body102. The adhesion layer may be capable of bonding to the non-oxidematerial of the dielectric body 102 while also being capable of forminga contact bond with the optical window 104. For example, the dielectricbody 102 may be formed of silicon and the example vapor cell 100 mayinclude an adhesion layer that includes silicon oxide (e.g., SiO₂,SiO_(x), etc.) on the dielectric body 102. This adhesion layer definesthe surface 106 of the dielectric body 102 and is capable of formingsiloxane bonds when processed according the methods of manufacturedescribed herein.

The dielectric body 102 includes a surface 106 that defines an opening108 to a cavity 110 in the dielectric body 102. The surface 106 may be aplanar surface, as shown in FIGS. 1A-1B, although other surfaces arepossible (e.g., curved). The opening 108 may be any type of opening thatallows access to an internal volume of the cavity 110 and may have anyshape (e.g., circular, square, hexagonal, oval, etc.). Such access mayallow a vapor (or a source of the vapor) to be disposed into the cavity110 during manufacture of the vapor cell 100. The cavity 110 extendsfrom the surface 106 into the dielectric body 104 and stops beforeextending completely through the dielectric body 104. The cavity 110 mayhave a uniform cross-section along its extension through the dielectricbody. However, in some variations, the cross-section of cavity 110 mayvary along its extension.

The example vapor cell 100 includes a vapor (not shown) in the cavity110 of the dielectric body 102. The vapor may include constituents suchas a gas of alkali-metal atoms, a noble gas, a gas of diatomic halogenmolecules, or a gas of organic molecules. For example, the vapor mayinclude a gas of alkali-metal atoms (e.g., K, Rb, Cs, etc.), a noble gas(e.g., He, Ne, Ar, Kr, etc.), or both. In another example, the vapor mayinclude a gas of diatomic halogen molecules (e.g., F₂, Cl₂, Br₂, etc.),a noble gas, or both. In yet another example, the vapor may include agas of organic molecules (e.g., acetylene), a noble gas, or both. Othercombinations for the vapor are possible, including other constituents.

The example vapor cell 100 may also include a source of the vapor in thecavity 110 of the dielectric body 102. The source of the vapor maygenerate the vapor in response to an energetic stimulus, such as heat,exposure to ultraviolet radiation, and so forth. For example, the vapormay correspond to a gas of alkali-metal atoms and the source of thevapor may correspond to an alkali-metal mass sufficiently cooled to bein a solid or liquid phase when disposed into the cavity 110. In someimplementations, the source of the vapor resides in the cavity of thedielectric body, and the source of the vapor includes a liquid or solidsource of the alkali-metal atoms configured to generate a gas of thealkali-metal atoms when heated.

The example vapor cell 100 additionally includes the optical window 104.As shown in FIG. 1B, the optical window 104 covers the opening 108 ofthe cavity 110 and has a surface 112 bonded to the surface 106 of thedielectric body 102. This bonding forms a seal around the opening 108.The seal includes metal-oxygen bonds formed by reacting a firstplurality of hydroxyl ligands on the surface 106 of the dielectric body102 with a second plurality of hydroxyl ligands on the surface 112 ofthe optical window 104. If one or both of the dielectric body 102 (or anadhesion layer thereon) and the optical window 104 include siliconoxide, the metal-oxide bonds may include siloxane bonds (i.e., Si—O—Si).However, other types of metal-oxygen bonds are possible, includinghybrid oxo-metal bonds. For example, if the dielectric body 102 and theoptical window are both formed of sapphire (e.g., Al₂O₃), themetal-oxygen bonds may include oxo-aluminum bonds (e.g., Al—O—Al). Ifthe dielectric body 102 is formed of a glass that includes silicon oxideand the optical window 104 is formed of sapphire, the metal-oxygen bondsmay include silicon-oxo-aluminum bonds (e.g., Si—O—Al, Al—O—Si, etc.).

The optical window 104 may be formed of a material transparent toelectromagnetic radiation (e.g., laser light) used to probe the vaporsealed within the cavity 110 of the dielectric body 102. For example,the material of the optical window 104 may be transparent to infraredwavelengths of electromagnetic radiation (e.g., 700-1000 nm), visiblewavelengths of electromagnetic radiation (e.g., 400-7000 nm), orultraviolet wavelengths of electromagnetic radiation (e.g., 10-400 nm).Moreover, the material of the optical window 104 may be an insulatingmaterial having a high resistivity, e.g., ρ>10⁸ Ω·cm, and may alsocorrespond to a single crystal, a polycrystalline ceramic, or anamorphous glass. For example, the material of the optical window 104 mayinclude silicon oxide (e.g., SiO₂, SiO_(x), etc.), such as found withinquartz, vitreous silica, or a borosilicate glass. In another example,the material of the optical window 104 may include aluminum oxide (e.g.,Al₂O₃, Al_(x)O_(y), etc.), such as found in sapphire or analuminosilicate glass. In some instances, the material of the opticalwindow 104 is an oxide material such as magnesium oxide (e.g., MgO),aluminum oxide (e.g., Al₂O₃), silicon dioxide (e.g., SiO₂), titaniumdioxide (e.g., TiO₂), zirconium dioxide, (e.g., ZrO₂), yttrium oxide(e.g., Y₂O₃), lanthanum oxide (e.g., La₂O₃), and so forth. The oxidematerial may be non-stoichiometric (e.g., SiO_(x)), and may also be acombination of one or more binary oxides (e.g., Y:ZrO₂, LaAlO₃, etc.).In other instances, the material is a non-oxide material such as diamond(C), calcium fluoride (CaF), and so forth.

In many implementations, the surface 106 of the dielectric body 102 andthe surface 112 of the optical window 104 may have a surface roughnessR_(a), no greater than a threshold surface roughness. The thresholdsurface roughness may ensure that, during contact bonding, pathways arenot formed that leak through the seal. Such pathways, if present, mightallow contaminates to enter the cavity 110 and vapor to exit the vaporcell 100. In some variations, the threshold surface roughness is lessthan 50 nm. In some variations, the threshold surface roughness is lessthan 30 nm. In some variations, the threshold surface roughness is lessthan 10 nm. In some variations, the threshold surface roughness is lessthan 1 nm.

Although FIGS. 1A and 1B depict the example vapor cell 100 as having asingle optical window, two or more optical windows are possible for theexample vapor cell 100. Moreover, in some variations, the cavity 110 mayextend entirely through dielectric body 102. FIG. 2A presents anexploded view, in perspective, of an example vapor cell 200 having twooptical windows. The example vapor cell 200 may be analogous in manyfeatures to the example vapor cell 100 shown by FIGS. 1A-1B. FIG. 2Bpresents the example vapor cell 200 of FIG. 2A, but in which bothoptical windows are bonded to a dielectric body 202 of the example vaporcell 200. At least one of the bonds is a contact bond, such as describedin relation to the example vapor cell 100 of FIGS. 1A-1B. The examplevapor cell 200 includes the dielectric body 202 and a cavity 204 in thedielectric body 202. The cavity 204 extends completely through thedielectric body 202. A first surface 206 of the dielectric body 202defines a first opening 208 to the cavity 204, and a second surface 210of the dielectric body 202 defines a second opening 212 to the cavity204.

The example vapor cell 200 also includes a first optical window 214covering the first opening 208 of the cavity 204. The first opticalwindow 214 has a surface 216 bonded to the first surface 206 of thedielectric body 202 to form a first seal around the first opening 208.The example vapor cell 200 additionally includes a second optical window218 covering the second opening 212 of the cavity 204. The secondoptical window 218 has a surface 220 bonded to the second surface 210 ofthe dielectric body 202 to form a second seal around the second opening212. The second seal includes metal-oxygen bonds formed by reacting afirst plurality of hydroxyl ligands on the second surface 210 of thedielectric body 202 with a second plurality of hydroxyl ligands on thesurface 220 of the second optical window 218. A vapor or a source of thevapor (not shown) resides in the cavity 204 of the dielectric body 202.

The dielectric body 202 and the optical windows 214, 218 may sharefeatures in common with, respectively, the dielectric body 102 and theoptical window 104 described in relation to the example vapor cell 100of FIGS. 1A-1B. For example, the dielectric body 202 may be formed ofsilicon (Si), aluminum oxide (e.g., Al₂O₃), or a glass that includessilicon oxide (e.g., SiO₂, SiO_(x), etc.). In another example, one orboth of first and second optical windows 214, 218 may be formed of amaterial transparent to electromagnetic radiation (e.g., laser light)used to probe the vapor sealed within the cavity 204 of the dielectricbody 202. Other features and their combinations are possible. Similarly,the vapor and the source of the vapor may share features in common with,respectively, the vapor and the source of the vapor described inrelation to the example vapor cell 100 of FIGS. 1A-1B. For example, thevapor may include a gas of alkali-metal atoms, a noble gas, a gas ofdiatomic halogen molecules, a gas of organic molecules, or somecombination thereof. In another example, the source of the vapor mayreside in the cavity 204 of the dielectric body 202, and the source ofthe vapor may include a liquid or a solid source of alkali-metal atomsconfigured to generate a gas of the alkali-metal atoms when heated.Other features and their combinations are possible.

In implementations where the dielectric body 202 is formed of anon-oxide material, an adhesion layer may be disposed on the dielectricbody 202 to define the second surface 210 of the dielectric body 202.The adhesion layer may be capable of bonding to the non-oxide materialof the dielectric body 202 while also being capable of forming a contactbond with the surface 220 of the second optical window 218. For example,the dielectric body 202 may be formed of silicon and the example vaporcell 200 may include an adhesion layer that includes silicon oxide(e.g., SiO₂, SiO_(x), etc.) on the dielectric body 202. This adhesionlayer defines the second surface 210 of the dielectric body 202 and iscapable of forming siloxane bonds when processed according the methodsof manufacture described herein. In some implementations, the first sealcomprises metal-oxygen bonds formed by reacting a third plurality ofhydroxyl ligands on the first surface 206 of the dielectric body 202with a fourth plurality of hydroxyl ligands on the surface 216 of thefirst optical window 214. In these implementations, example vapor cell200 may include an adhesion layer disposed on the dielectric body 202 todefine the first surface 206 of the dielectric body 202 if thedielectric body is formed of a non-oxide material.

In some implementations, such as shown in FIGS. 2A-2B, the first andsecond surfaces 206, 210 of the dielectric body 202 are planar surfacesopposite each other, and the surface 216 of the first optical window 214and the surface 220 of the second optical window 218 are planarsurfaces. In some implementations, the second surface 210 of thedielectric body 202 and the surface 220 of the second optical window 218have a surface roughness, R_(a), no greater than a threshold surfaceroughness. In some variations, the threshold surface roughness is lessthan 50 nm. In some variations, the threshold surface roughness is lessthan 30 nm. In some variations, the threshold surface roughness is lessthan 10 nm. In some variations, the threshold surface roughness is lessthan 1 nm. In further implementations, the threshold surface roughnessis a second threshold surface roughness, and the first surface 206 ofthe dielectric body 202 and surface 216 of the first optical window 214have a surface roughness, R_(a), no greater than a first thresholdsurface roughness. The first threshold surface roughness need not be thesame as the second threshold surface roughness.

In some implementations, the first seal includes an anodic bond betweenthe first surface 206 of the dielectric body 202 and the surface 216 ofthe first optical window 214. In some implementations, the dielectricbody 202 is formed of a glass comprising silicon oxide (e.g., SiO₂,SiO_(x), etc.) and the first optical window 214 includes silicon oxide(e.g., SiO₂, SiO_(x), etc.). In these implementations, the example vaporcell 200 includes a layer of silicon disposed between the first surface206 of the dielectric body 202 and the surface 216 of the first opticalwindow 214. The first seal includes an anodic bond between the layer ofsilicon and one or both of the first surface 206 of the dielectric body202 and the surface 216 of the first optical window 214.

In some implementations, the dielectric body 202 is formed of a glasscomprising silicon oxide (e.g., SiO₂, SiO_(x), etc.) and the firstoptical window 214 includes silicon oxide (e.g., SiO₂, SiO_(x), etc.).In such cases, the example vapor cell 200 includes a fired layer ofglass frit bonding the first surface 206 of the dielectric body 202 tothe surface 216 of the first optical window 214. The fired layer ofglass frit defines the first seal.

FIG. 3 presents an example vapor cell 300, shown from perspective, top,and side views, that has a dielectric body 302 formed of silicon and twooptical windows 304, 306 formed of glass. The example vapor cell 300 maybe a single vapor cell, or alternatively, be part of an array of suchcells (e.g., manufactured using large substrates or wafers). The glassincludes silicon oxide, although a composition of the glass may be thesame or different for the two optical windows 304, 306. The two opticalwindows 304, 306 cover respective openings to a cavity 308 in thedielectric body 302. In particular, a first optical window 304 is bondedto the dielectric body 302 to define a first seal, and a second opticalwindow 306 is bonded to the dielectric body 302 to define a second seal.The first and second seals assist in containing a vapor (or a source ofthe vapor) within the cavity 308 of the dielectric body 302. An adhesionlayer 310 is disposed along an interface 312 between the second opticalwindow 306 and the dielectric body 302. The adhesion layer 310 is formedof silicon dioxide (e.g., single-crystal quartz or vitreous silica) andallows a contact bond between the dielectric body 302 and the secondoptical window 306. The contact bond helps to define the second seal andincludes siloxane bonds (i.e., Si—O—Si) between the adhesion layer 310and the second optical window 306. The first seal may include an anodicbond between the dielectric body 302 and the first optical window 304.Alternatively, a fired layer of glass frit may bond the dielectric body302 to the first optical window 304 to form the first seal.

FIG. 4 presents an example vapor cell 400, shown from perspective, top,and side views, that has a dielectric body 402 formed of glass and twooptical windows 404, 406 formed of glass. Similar to FIG. 3, the examplevapor cell 400 may be a single vapor cell, or alternatively, be part ofan array of such cells (e.g., manufactured using large substrates orwafers). The glass includes silicon oxide, although a composition of theglass may be the same or different for any combination of the dielectricbody 402 and the two optical windows 404, 406. The two optical windows404, 406 cover respective openings to a cavity 408 in the dielectricbody 402. In particular, a first optical window 404 is bonded to thedielectric body 402 to define a first seal, and a second optical window406 is bonded to the dielectric body 402 to define a second seal. Thefirst and second seals assist in containing a vapor (or a source of thevapor) within the cavity 408 of the dielectric body 402. A layer ofsilicon 410 is disposed along an interface 412 between the first opticalwindow 404 and the dielectric body 402. The layer of silicon 410 allowsan anodic bond between the first optical window 404 and the dielectricbody 402, which helps to define the first seal. A contact bond definesthe second seal and includes siloxane bonds (i.e., Si—O—Si) between thedielectric body 402 and the second optical window 406.

Although FIGS. 1A-4 depict square vapor cells with circular cavities,other geometries are possible for the vapor cells and their respectivecavities. For example, FIGS. 5A-5C present, in perspective view,alternative geometries for example vapor cells 500, 520, 540. In FIG.5A, the example vapor cell 500 includes a circular dielectric body 502and circular optical windows 504, 506. The circular dielectric body 502includes a square wall 508 to define a cavity 510 therein. In somevariations, the circular dielectric body 502 is formed of silicon andmay be about 0.5 mm thick. A diameter of the circular dielectric body502 may be about 1.5 mm. In some variations, the circular opticalwindows 504, 506 may be about 0.05 mm-0.30 mm thick. The circularoptical windows 506, 506 may share a diameter in common with thecircular dielectric body 502. In some variations, the square wall 508may have an edge length of about 0.9 mm.

In FIG. 5B, the example vapor cell 520 includes a square dielectric body522 and square optical windows 524, 526. The square dielectric body 522includes a circular wall 528 to define a cavity 530 therein. In somevariations, the square dielectric body 522 is formed of silicon and maybe about 0.5 mm thick. An edge length of the square dielectric body 522may be about 1.5 mm. In some variations, the square optical windows 524,526 may be about 0.05 mm-0.30 mm thick. The square optical windows 524,526 may share an edge length in common with the square dielectric body522. In some variations, the circular wall 528 may have a diameter ofabout 0.9 mm.

In FIG. 5C, the example vapor cell 540 includes a circular dielectricbody 542 and circular optical windows 544, 546. The circular dielectricbody 542 includes circular wall 548 to define a cavity 550 therein. Insome variations, the circular dielectric body 542 is formed of siliconand may be about 0.5 mm thick. A diameter of the circular dielectricbody 542 may be about 1.5 mm. In some variations, the circular opticalwindows 544, 546 may be about 0.05 mm-0.30 mm thick. The circularoptical windows 544, 546 may share a diameter in common with thecircular dielectric body 542. In some variations, the circular wall 548may have a diameter of about 0.9 mm.

The vapor cells described in relation to FIGS. 1A-5C can be manufacturedusing the methods described herein. The methods of manufacture canproduce vapor cell sizes much smaller than a wavelength ofelectromagnetic radiation to be detected or measured. For atom-basedmagnetometry, the methods may include a low temperature method forsealing the vapor cells that allows the vapor cells to be coated withanti-relaxation coatings. The methods include constructing a frame orbody of vapor cells using laser machining or a lithographic technique,followed by an etching method such as deep reactive ion etching (DRIE)to make individual structures or many structures on a chip. Materialsused for the frame (e.g., silicon) are coated prior to chemical or lasermachining, such as with a silicon oxide material (e.g., SiO₂, SiO_(x),etc.) or another adhesion layer. Next, an optical window that cantransmit the desired electromagnetic waves is anodically bonded to theframe (or bonded in some other way, such as glass frit bonding). Thisbonding operation produces a strong bond that is leak-tight for highvacuum. However, in certain cases, the anodic bonding operation may alsoproduce outgassing. For example, in methods such as those used toconstruct chip-scale atomic clocks, anodic bonding is used to fastenoptical windows onto a frame or body. In these methods, a large amountof outgassing may occur because of the relatively high temperatures andhigh voltages required, which in turn, strongly drives ion diffusion.The outgassing causes the pressure of unwanted gases in the vapor cellto rise to unacceptably high levels, which may degrade the performanceof the vapor cell.

To help avoid or mitigate outgassing, the methods presented hereinproduce a seal using a contact-bonding operation. In the contact-bondingoperation, an adhesion layer and optical window may be prepared usingplasma activation. In some variations, a frame or body with an opticalwindow is placed in an atmosphere of a gas, thereby allowing the gas tofill the vapor cell(s) to a desired pressure. A contact bond is thenmade by pressing the plasma activated sides of the pieces together toform the seal. Other filling techniques, however, are possible. Forexample, an alkali metal mass may be dotted onto an anodically-bondedwindow and then sealed under high vacuum conditions in the vapor cellwith a contact-bonding operation. Alternatively, a variouslaser-activated source of a vapor of gas may be placed in the vapor celland then sealed under high vacuum conditions by a contact-bondingoperation. The methods presented herein are well-suited for producingvapor cells coated with anti-relaxation coatings for magnetometry sincethe contact-bonding operation, which may correspond to a finaloperation, occurs at low temperatures.

The methods of manufacturing described herein can fabricate stemlessvapor cells with high purity gas samples by combining strong bondingmethods (e.g., anodic bonding in atmosphere) with contact bonding invacuum. The methods are conducive for large scale manufacturing becausemany vapor cells can be made at the same time on a large substrate(e.g., a wafer) and then cut out using either a laser machining processor a dicing saw. In some variations, the vapor cells have dielectricbodies formed of silicon or a glass that includes silicon oxide (e.g.,SiO₂, SiO_(x), etc.). The dielectric body is machined either chemicallyor with a laser to create cavities for the filling vapor. If thecavities created pass through the dielectric body, then a first opticalwindow is anodically bonded to one side of the dielectric body. It isunnecessary to anodically bond the first optical window if the hole doesnot pass through the frame. Glass frit bonding or another type ofbonding may be used for the first optical window. Anodic bonding workswell for glass to silicon bonds, and glass frit bonding works well forglass on glass bonds.

In certain of the methods, the dielectric body is prepared with a thinlayer (e.g., about 500 nm) of silicon oxide (e.g., SiO₂, SiO_(x), etc.)when formed of silicon so that a contact bond can be made between anoptical window (or a second optical window) and the dielectric body. Thedielectric body may also be prepared with the thin layer of siliconoxide if formed of another material similar to silicon (e.g., anon-oxide material). Following this preparation, a contact bond is madein a pure atmosphere of the vapor or gas that fills the vapor cell(e.g., a gas of alkali-metal atoms that includes cesium or rubidium). Inmany implementations, the contact bond is done at low temperatures(e.g., less than 100° C.) and zero applied voltage to prevent outgassingof undesirable gasses into the cavity of the vapor cell.

The contact bond is formed using metal-oxygen bonding between a surfaceof the dielectric body and a mating surface of an optical window. Forvariations in which the dielectric body is formed of silicon (or a glassthat includes silicon oxide), and the optical window is formed of glassthat includes silicon oxide, the contact bonding may be formed usingsiloxane bonds (i.e., Si—O—Si). A reaction of the contact-bond formationprocess may be represented by:Si—OH+HO—Si⇄Si—O—Si+H₂OThe reaction is reversible, so in some instances, it is desirable toremove the water molecules generated from the reaction. Otherwise, thenewly-formed siloxane bonds are at risk in being hydrolyzed back intosilanol bonds (i.e., Si—OH).

Water molecules generated by contact-bonding can be removed by reactionswhose products are solid at room temperature. In some implementations,the water molecules are reacted with the vapor in the vapor cell. Forexample, the vapor may be a gas of cesium atoms, and the water moleculesmay be reacted with a portion of the gas to form solids, such as Cs₂O(T_(melt)≅340° C.), CsOH (T_(melt)≅272° C.), or CsH (T_(melt)≅170° C.).In some implementations, the water molecules are reacted with adesiccant material that resides in the cavity of the dielectric body(e.g., as a coating, a dotted mass, etc.). The desiccant material may beinert to the vapor in the vapor cell. For example, the vapor may be agas of diatomic halogen molecules (e.g., chlorine gas), and the watermolecules may be reacted with an anhydrous chloride salt (e.g., LaCl₃)to form products, such as hydrated salts or oxyhydroxide compounds(e.g., LaCl₃.xH₂O, LaOCl, etc.).

Although the representative reaction is presented in the context ofsilicon as the participating metal atoms, other metal atoms arepossible. For example, if the dielectric body is formed of aluminumoxide (e.g., single-crystal sapphire) and the optical window is alsoformed of aluminum oxide (e.g., Al₂O₃ polycrystalline ceramic), thecontact-bond formation process may utilize aluminum as the metal atomand form oxo-aluminum bonds (e.g., Al—O—Al). Mixtures of metals are alsopossible. For example, if the dielectric body is formed of zirconiumoxide and the optical window is formed of magnesium oxide, thecontact-bond formation process may utilize zirconium and magnesium asthe metal atoms and form zirconium-oxo-magnesium bonds.

In general, for a dielectric body formed of a material that includes afirst metal, M₁, and an optical window formed of a material thatincludes a second metal, M₂, a reaction of the contact-bond formationprocess may be represented by:M₁-OH+HO-M₂⇄M₁-O-M₂+H₂O

Here, a hydroxyl ligand (i.e., OH) is coordinated to each of the metalatoms, M₁ and M₂, and the hydroxyl ligands condense into an oxo ligand(O) during formation of the metal-oxygen bond (i.e., M₁-O-M₂). A watermolecule is liberated as a by-product of this condensation process.Although the reaction suggests a single hydroxyl ligand per metal atom,other numbers of hydroxyl ligands may be coordinated to each of themetal atoms, M₁ and M₂.

In many variations, the condensation of hydroxyl ligands occurs at roomtemperature upon contact of a surface of the dielectric body with amating surface of the optical window. However, in some variations, heatmay be applied to initiate and/or complete the formation of the contactbond. The heat may also strengthen the contact bond. For example, heatmay be applied to one or both of the dielectric body and the opticalwindow to increase their respective temperatures to a processingtemperature. The processing temperature may facilitate formation of thecontact bond. In some variations, the processing temperature is nogreater than 250° C. In some variations, the processing temperature isno greater than 120° C. In some variations, the processing temperatureis no greater than 75° C.

After the contact bond is formed, the vapor cell can abe coated with amaterial to protect the vapor cell such as parylene or an epoxy. Thevapor cell can also be fiber coupled by attaching a fiber to thewindow(s) either directly or with a GRIN lens.

As described above, the methods of manufacture allow the construction ofsmall, stemless vapor cells that can be used for atom-based sensing.Such sensors may be used for metrological purposes. A sub-wavelengthvapor cell for sensing MHz to THz electromagnetic fields reduces aphysical cross-section for the scattering of electromagnetic fieldsmeasured by the vapor cell. The cross-section scales like the ratio ofthe vapor cell dimensions divided by lambda to the third power. (Lambdacorresponds to a wavelength of electromagnetic radiation measured by thevapor cell.) Moreover, stems of vapor cells that contain vapor (e.g. agas of alkali-metal atoms) can cause a distortion of the measuredelectromagnetic fields. Vapor cells that lack stems thus offer improvedmeasurement capabilities relative to conventional vapor cells, whichinclude stems. Stemless vapor cells are also conducive for chip-scaledevices that utilize atom-based sensing.

The methods of manufacture also allow multiple vapor cells to beconnected together or arranged in an array to make multiple simultaneousmeasurements in a region of space. For example, they could be arrangedin a planar array so that an electromagnetic field could becharacterized in the sensor plane. 3-dimensional arrays are alsopossible. These features are enabled by the dielectric nature of theprobes since the vapor cells minimally interact with each other—theyhave low scattering cross-sections. The light can be transported to thevapor cells through optical waveguides, such as fiber, in parallel orseries, but have to be readout independently (the signal light has to besplit off at each vapor cell to give a measurement that reflects theabsorption or dispersive signal associated with the individual vaporcell). In essence, this is a multipixel array, but the transparentnature of vapor cells makes 3-dimensional imaging possible. Thick cellscan be manufactured by this method by stacking unit cells together or tomake unique shapes, such as taking anodically bonded glass+frames (lasercut together), stacking them and anodically bonding several together oneat a time, and then capping the structure with a contact bond.

The methods of manufacturing may include a contact-bonding operation tofabricate a vapor cell. The contact-bonding operation may includeprocesses to chemically alter a surface of a dielectric body (or frame)and a mating surface of an optical window. The altered surfaces are thencontacted together to form metal-oxygen bonds (e.g., siloxane bonds)that create a seal. In some implementations, a method of manufacturing avapor cell includes obtaining a dielectric body that has a surface thatdefines an opening to a cavity in the dielectric body. The method alsoincludes obtaining an optical window that has a surface (or matingsurface). One or both of the surfaces of the dielectric body and theoptical window may have a surface roughness, R_(a), no greater than athreshold surface roughness, such as described in relation to the vaporcell 100 of FIGS. 1A-1B. For example, the threshold surface roughnessmay be 1 nm. In some instances, the surface of the dielectric body andthe surface of the optical window are planar surfaces.

In the method, the surfaces of the dielectric body and the opticalwindow are altered to include, respectively, a first plurality ofhydroxyl ligands and a second plurality of hydroxyl ligands. The methodadditionally includes disposing a vapor, or a source of the vapor, intothe cavity. The altered surface of the dielectric body is contacted tothe altered surface of the optical window to form a seal around theopening to the cavity. The seal, which may define part or all of acontact bond, includes metal-oxygen bonds formed by reacting the firstplurality of hydroxyl ligands with the second plurality of hydroxylligands during contact of the altered surfaces. In some variations,contacting the altered surfaces includes covering the opening of thecavity with the optical window to enclose the vapor or the source of thevapor in the cavity.

The dielectric body and optical window may be formed of materialstransparent to electric fields (or electromagnetic radiation) measuredby the vapor cell. Examples of such materials are described in relationto the dielectric body 102 and optical window 104 of FIGS. 1A-1B. Forexample, the dielectric body may be formed of silicon or a glasscomprising silicon oxide (e.g., SiO₂, SiO_(x), etc.). The optical windowmay include silicon oxide (e.g., SiO₂, SiO_(x), etc.). In theseexamples, the metal-oxygen bonds may include siloxane bonds. Inimplementations where the dielectric body is formed of a non-oxidematerial, the method may include forming an adhesion layer on thedielectric body that defines the surface of the dielectric body. Forexample, if the dielectric body is formed of silicon, the adhesion layermay include silicon oxide (e.g., SiO₂, SiO_(x), etc.), such as describedin relation to the vapor cell 300 of FIG. 3.

As described above, the vapor or the source of the vapor is disposedinto the cavity while conducting the method. The vapor may include a gasof alkali-metal atoms, a gas of diatomic halogen molecules, a gas oforganic molecules, a noble gas, or some combination thereof, such asdescribed in relation to the vapor cell 100 of FIGS. 1A-1B. In someimplementations, disposing the vapor or the source of the vapor includesexposing the cavity to a vacuum environment comprising a gas ofalkali-metal atoms. In some implementations, disposing the vapor or thesource of the vapor includes disposing a solid or liquid source ofalkali-metal atoms into the cavity. In these implementations, the methodincludes heating the solid or liquid source of alkali-metal atoms togenerate a gas of the alkali-metal atoms after contacting. However,other types of energetic stimulus are possible for the solid or liquidsource of alkali-metal atoms (e.g., exposure to laser light, ultravioletradiation, etc.).

In some implementations, altering the surfaces includes activating oneor both of the surfaces of the dielectric body and the optical window byexposing the respective surfaces to a plasma. Such exposure may increasea surface energy of one or both the surfaces of the dielectric body andthe optical window and chemically prepare the surfaces for subsequentcontact bonding. Further chemical preparation may occur by washing oneor both of the activated surfaces of the dielectric body and the opticalwindow in a basic aqueous solution. Contact with the basic aqueoussolution may coordinate metal atoms on the surfaces of the dielectricbody and the optical window with hydroxyl ligands.

In some implementations, contacting the altered surfaces includespressing the altered surfaces of the dielectric body and the opticalwindow against each other. In some instances, the seal is formed uponcontact of the altered surfaces. In other instances, pressing mayinvolve contacting the altered surfaces with a pressure up to 2 MPa. Insome implementations, the method includes heating one or both thedielectric body and the optical window after contacting the alteredsurfaces. For example, one or both of the dielectric body and theoptical window may be heated to a temperature ranging from 100° C.-250°C. after contacting the altered surfaces. While heating, the dielectricbody and the optical window may be clamped together to hold the alteredsurfaces in contact.

In some implementations, obtaining the dielectric body includes removingmaterial from the dielectric body to form the cavity. Removing materialmay include machining material from the surface of the dielectric bodywith a laser. Removing material may also include etching material fromthe surface of the dielectric body. Such etching may involve one or bothof a dry or wet etching process. Other types of subtractive processesare possible for the operation of removing material (e.g., ablation,grinding, polishing, etc.).

The methods of manufacturing described herein may be used to fabricatevapor cells with more than one optical window. In some implementations,a method of manufacturing a vapor cell that has at least two opticalwindows includes obtaining a dielectric body. The dielectric body has acavity therein and includes a first surface that defines a first openingto the cavity and a second surface that defines a second opening to thecavity. The method also includes obtaining a first optical window thathas a surface. The surface of the first optical window is bonded to thefirst surface of the dielectric body to form a first seal around thefirst opening to the cavity. In some implementations, bonding thesurfaces includes covering the first opening of the cavity with thefirst optical window.

The method additionally includes obtaining a second optical window thatincludes a surface. The second surface of the dielectric body and thesurface of the second optical window are altered to include,respectively, a first plurality of hydroxyl ligands and a secondplurality of hydroxyl ligands. The method further includes disposing avapor or a source of the vapor into the cavity through the secondopening. The altered second surface of the dielectric body is contactedto the altered surface of the second optical window to form a secondseal around the second opening to the cavity. The second seal includesmetal-oxygen bonds formed by reacting the first plurality of hydroxylligands with the second plurality of hydroxyl ligands during contact ofthe altered surfaces. In some implementations, contacting the alteredsurfaces includes covering the second opening of the cavity with thesecond optical window to enclose the vapor or the source of the vapor inthe cavity.

One or both of the second surface of the dielectric body and the surfaceof the second optical window may have a surface roughness, R_(a), nogreater than a threshold surface roughness, such as described inrelation to the vapor cell 200 of FIGS. 2A-2B. For example, thethreshold surface roughness may be 1 nm. In some implementations, thethreshold surface roughness is a second threshold surface roughness andone or both of the first surface of the dielectric body and the surfaceof the first optical window may have a surface roughness, R_(a), nogreater than a first threshold surface roughness. The first thresholdsurface roughness need not be the same as the second threshold surfaceroughness. In some variations, the first and second surfaces of thedielectric body are planar surfaces opposite each other, and the surfaceof the first optical window and the surface of the second optical windoware planar surfaces.

In the method, the surfaces of the dielectric body and the opticalwindow are altered to include, respectively, a first plurality ofhydroxyl ligands and a second plurality of hydroxyl ligands. The methodadditionally includes disposing a vapor, or a source of the vapor, intothe cavity. The altered surface of the dielectric body is contacted tothe altered surface of the optical window to form a seal around theopening to the cavity. The seal, which may define part or all of acontact bond, includes metal-oxygen bonds formed by reacting the firstplurality of hydroxyl ligands with the second plurality of hydroxylligands during contact of the altered surfaces. In some variations,contacting the altered surfaces includes covering the opening of thecavity with the optical window to enclose the vapor or the source of thevapor in the cavity.

The dielectric body, the first optical window, and the second opticalwindow may be formed of materials highly transparent to electric fields(or electromagnetic radiation) measured by the vapor cell. Examples ofsuch materials are described in relation to the dielectric bodies 102,202 and optical windows 104, 214, 218 of FIGS. 1A-2B. For example, thedielectric body may be formed of silicon or a glass comprising siliconoxide (e.g., SiO₂, SiO_(x), etc.), and one or both of the first andsecond optical windows may include silicon oxide (e.g., SiO₂, SiO_(x),etc.). In these examples, the metal-oxygen bonds may include siloxanebonds. In implementations where the dielectric body is formed of anon-oxide material, the method may include forming an adhesion layer onthe dielectric body that defines the second surface of the dielectricbody. For example, if the dielectric body is formed of silicon, theadhesion layer may include silicon oxide (e.g., SiO₂, SiO_(x), etc.),such as described in relation to the vapor cell 300 of FIG. 3.

As described above, the vapor or the source of the vapor is disposedinto the cavity while conducting the method. The vapor may include a gasof alkali-metal atoms, a gas of diatomic halogen molecules, a gas oforganic molecules, a noble gas, or some combination thereof, such asdescribed in relation to the vapor cells 100, 200 of FIGS. 1A-2B. Insome implementations, disposing the vapor or the source of the vaporincludes exposing the cavity to a vacuum environment comprising a gas ofalkali-metal atoms. In some implementations, disposing the vapor or thesource of the vapor includes disposing a solid or liquid source ofalkali-metal atoms into the cavity through the second opening. In theseimplementations, the method includes heating the solid or liquid sourceof alkali-metal atoms to generate a gas of the alkali-metal atoms aftercontacting. However, other types of energetic stimulus are possible forthe solid or liquid source of alkali-metal atoms (e.g., exposure tolaser light, ultraviolet radiation, etc.).

In some implementations, altering the surfaces includes activating oneor both of the second surface of the dielectric body and the surface ofthe second optical window by exposing the respective surfaces to aplasma. Such exposure may increase a surface energy of one or both thesecond surfaces of the dielectric body and the surface of the secondoptical window and chemically prepare the surfaces for subsequentcontact bonding. Further chemical preparation may occur by washing oneor both of the activated surfaces of the dielectric body and the secondoptical window in a basic aqueous solution. Contact with the basicaqueous solution may coordinate metal atoms on the surfaces of thedielectric body and the optical window with hydroxyl ligands.

In some implementations, contacting the altered surfaces includespressing the altered second surface of the dielectric body and thealtered surface of the second optical window against each other. In someinstances, the second seal is formed upon contact of the alteredsurfaces. In other instances, pressing may involve contacting thealtered surfaces with a pressure up to 2 MPa. In some implementations,the method includes heating one or both the dielectric body and thesecond optical window after contacting the altered surfaces. Forexample, one or both of the dielectric body and the second opticalwindow may be heated to a temperature ranging from 100° C.-250° C. aftercontacting the altered surfaces. While heating, the dielectric body andthe second optical window may be clamped together to hold the alteredsurfaces in contact.

In the method, the first optical window may be contact bonded to thedielectric body in addition to the second optical window. In someimplementations, bonding the surface of the first optical windowincludes altering the first surface of the dielectric body and thesurface of the first optical window to include, respectively, a thirdplurality of hydroxyl ligands and a fourth plurality of hydroxylligands. The altered first surface of the dielectric body is contactedto the altered surface of the first optical window to form the firstseal around the first opening to the cavity. The first seal includesmetal-oxygen bonds formed by reacting the third plurality of hydroxylligands with the fourth plurality of hydroxyl ligands during contact ofthe altered surfaces. In these implementations, contacting the alteredfirst surface of the dielectric body to the altered surface of the firstoptical window may include pressing the altered surfaces against eachother. In some instances, the first seal is formed upon contact of thealtered surfaces. In other instances, pressing may involve contactingthe altered surfaces with a pressure up to 2 MPa. In some variations,one or both the dielectric body and the first optical window are heatedafter the altered surfaces are contacted. For example, one or both ofthe dielectric body and the first optical window may be heated to atemperature ranging from 100° C.-250° C. after contacting the alteredsurfaces. While heating, the dielectric body and the first opticalwindow may be clamped together to hold the altered surfaces in contact.

The method also allows the first optical window to be contact bonded tothe dielectric body with a bond other than a contact bond. For example,the dielectric body may be formed of silicon and the first opticalwindow may include silicon oxide (e.g., SiO₂, SiO_(x), etc.). In thisexample, bonding the surface of the first optical window includesanodically bonding the surface of the first optical window to the firstsurface of the dielectric body to form the first seal. The example vaporcell 300 of FIG. 3 may correspond to a vapor cell manufactured by thisvariation of the method. In another example, the dielectric body may beformed of a glass comprising silicon oxide and the first optical windowmay include silicon oxide (e.g., SiO₂, SiO_(x), etc.). In this secondexample, the method includes depositing a layer of silicon on the firstsurface of the dielectric body, and bonding the surface of the firstoptical window includes anodically bonding the layer of silicon to thesurface of the first optical window to form the first seal. The examplevapor cell 400 of FIG. 4 may correspond to a vapor cell manufactured bythis variation of the method. In yet another example, the dielectricbody may be formed of a glass comprising silicon oxide and the firstoptical window may include silicon oxide (e.g., SiO₂, SiO_(x), etc.). Inthis third example, bonding the surface of the first optical windowincludes applying a glass frit to one or both of the first surface ofthe dielectric body and the surface of the first optical window. Thefirst surface of the dielectric body is contacted to the surface of thefirst optical window, and at least one of the glass frit, the dielectricbody, or the first optical window are heated to a firing temperature toform the first seal. Although the forgoing examples have been presentedin the context of silicon and silicon oxide, other materials arepossible for the method.

In some implementations, obtaining the dielectric body includes removingmaterial from the dielectric body to form the cavity. Removing materialmay include machining material from the surface of the dielectric bodywith a laser. Removing material may also include etching material fromthe surface of the dielectric body. Such etching may involve one or bothof a dry or wet etching process. Other types of subtractive processesare possible for the operation of removing material (e.g., ablation,grinding, polishing, etc.).

EXAMPLES

The methods of manufacturing vapor cells may be described by thefollowing examples. However, examples are for purposes of illustrationonly. It will be apparent to those skilled in the art that manymodifications, both to materials and methods, may be practiced withoutdeparting from the scope of the disclosure.

Example 1

A p-type silicon wafer was obtained with a double-sided polish and an<100> orientation. The silicon wafer had a diameter of 4-inches and was500 μm thick with a surface roughness, R_(a), no greater than 1 nm oneach side. Electrical properties of the silicon wafer included aresistance that ranged from 0.1 Ω-cm to 0.3 Ω-cm. A glass wafer formedof borosilicate glass was also obtained from Schott. The glass wafer wasa MEMpax wafer having a diameter of 4 inches and a thickness of 300 μm.The surface roughness was less than 0.5 nm.

The silicon and glass wafers were inspected in preparation for anodicand contact bonding. In particular, the wafers were visually inspectedfor chips, micro-cracks, and scratches. The wafers were also verified tohave a surface roughness less than 1 nm. A 500-nm layer of SiO₂ wasgrown on both sides of the silicon wafer using a wet growth process inan oxidation furnace. The temperature of the oxidation furnace was setto about 1100° C. and the processing time of the silicon wafer was about40 min. A thickness uniformity of the silicon wafer (with the SiO₂layers) was verified to be within 500±6 nm over its 4-inch diameterarea. The surface roughness was also verified to be less than 1 nm.

Multiple silicon chips were cut from the silicon wafer using either aProtolaser U3 micro-laser tool, a Protolaser R micro-laser tool, or aDISCO DAD 3240 dicing saw. Each silicon chip had dimensions of 10 mm×20mm. Nine holes were subsequently machined through each of the siliconchips using the Protolaser U3 micro-laser tool or the Protolaser Rmicro-laser tool. The holes were each circular with a 1-mm diameter orsquare with a 1-mm edge length. In some cases, combinations of circlesand holes were machined in a silicon chip. The silicon chips wereinspected visually with 5× and 10× magnification loupes for cracks orchips that might have occurred during cutting. Silicon chips with zeroor minimal surface defects were selected for subsequent vapor-cellfabrication.

The selected silicon chips were then cleaned with methanol andisopropanol using cotton swabs and optical tissue paper. Next, thesilicon chips were submerged in a buffered oxide etch (BOE) solutionhaving a 10:1 volume ratio and an etch rate of 55 nm/min at roomtemperature. The buffered oxide etch solution contained hydrofluoricacid buffered with ammonium fluoride. The silicon chips were submergedfor at least 11 minutes to remove the 500-nm layer of SiO₂ from thesurface of each side of the silicon chips. After being removed from thebuffered oxide etch, the silicon chips were visually inspected. Ifembedded material from the cutting process was found on a silicon chip,the silicon chip was discarded. If regions of SiO₂ remained on a siliconchip, the silicon chip was re-submerged in the buffered oxide etchsolution, removed, and then re-inspected. Silicon chips with both sidesfree of the 500-nm layer of SiO₂ were selected for final cleaning andthe growth of a 100-nm SiO₂ layer.

The selected silicon chips were then cleaned with acetone andisopropanol using cotton swabs and optical tissue paper. An ultrasoniccleaner was optionally used to assist the cleaning process by agitatingbaths of acetone or isopropanol in which the selected silicon chips weresubmerged. A 100-nm layer of SiO₂ was then grown on one side of thesilicon chips. The temperature of the oxidation furnace was set to aminimum of 600° C. to obtain a surface roughness no greater than 1 nmfor the 100-nm layer of SiO₂. A thickness uniformity of the 100-nm SiO₂layer was verified to be within 100±6 nm over an area of a silicon chip.Silicon chips failing the uniformity criterion were discarded.

Silicon chips with the 100-nm SiO₂ layer were then cleaned with methanoland isopropanol using cotton swabs and optical tissue paper to eliminateloose residues on their surfaces (e.g., such as due to handling). Thesilicon chips were subsequently deep-cleaned with acetone andisopropanol using cotton swabs and optical tissue paper. A lowmagnification loupe (e.g., 10×) was used during the deep cleaningprocess for a first visual inspection followed by a high magnificationmicroscope (e.g., 50×-200×) for a second visual inspection. Siliconchips passing the second vision inspection were placed in a bath ofacetone for ultrasonic cleaning at 40 kHz (e.g., in a Branson UltrasonicCleaner CPX-952-117R). For example, the silicon chips could be placed ina glass beaker of acetone and cleaned ultrasonically for 20 minutes atroom temperature. After ultrasonic cleaning, the silicon chips weredried with particulate-free compressed air and stored in an air-tightcontainer until needed for bonding.

Separately, a dicing saw was used to cut the glass wafers into suitablesizes for bonding to the (stored) silicon chips. Two glass chips wereprepared for each silicon chip. If a glass chip was intended for ananodic bond, the glass chip was cut to have the same dimensions as thesilicon chip. However, if a glass chip was intended for a contact bond,the glass chip was cut to have longer dimensions than the silicon chip.For example, glass chips for anodic bonding had dimensions of 10 mm×20mm and glass chips for contact bonding had dimensions of 10 mm×35 mm.After cutting, each glass chip was inspected to ensure that its opticalclarity was not degraded (e.g., hazing), or that scratches or crackswere not present. Glass chips found to be acceptable were then cleanedwith acetone using cotton swabs and optical tissue paper. If necessary,the glass chips were placed in a glass beaker of acetone andultrasonically cleaned form 20 minutes at room temperature. Afterultrasonic cleaning, the glass chips were dried with particulate-freecompressed air and then stored in an air-tight container until neededfor bonding.

One silicon chip and one glass chip were then placed into an assemblyfor anodic bonding. For the silicon chip, the planar surface oppositethe planar surface defined by the 100-nm layer of SiO₂ participated inthe anodic bonding process. In the assembly, planar surfaces of siliconand glass chips were contacted to define an interface, and the interfacewas visually inspected to confirm that optical fringes were present. Thesilicon chip was then heated to a temperature of about 400° C. Afterthis temperature was reached, 600V was applied across the silicon andglass chips for about 15 minutes, which drove the formation of an anodicbond. The interface was inspected again to confirm the disappearance ofthe optical fringes, which indicated the anodic bond was complete. Next,the anodic bond was inspected for defects (e.g., bubbles, micro-cracks,unbonded areas, etc.). If 80% or more of an area around the holes wasfree of defects, the anodic bond was then further inspected for openchannels (e.g., from a hole to the environment, a hole to another hole,etc.). If an open channel was discovered, the anodically-bonded chipswere discarded as the anodic bond was not deemed leak-tight.

Bonded silicon and glass chips with leak-tight anodic bonds were cleanedin acetone and methanol. During this cleaning process, the unbondedsurface of the silicon chip was cleaned with acetone and methanol usingcotton swabs and optical tissue paper to eliminate any residues (e.g.,residues from a graphite plate of the assembly used to form the anodicbond). The unbonded surface of the silicon chip was then visuallyinspected to ensure defects (e.g., scratches, pitting, etc.) were notpresent that might compromise a soon-to-be formed contact bond. Theanodically-bonded chips were then individually cleaned. In particular,the anodically-bonded chips were placed individually (i.e., with noother chips) in a glass beaker of acetone and cleaned ultrasonically for20 minutes at room temperature. After ultrasonic cleaning, theanodically-bonded chips were dried with particulate-free compressed air.A low magnification loupe (e.g., 10×) was used for a first visualinspection of the anodically-bonded chips, followed by a highmagnification microscope (e.g., 50×-200×) for a second visualinspection. The first and second visual inspections were used to ensureno visual residues or deposits remained on the anodically-bonded chips.

The anodically-bonded chips—along with glass chips—were then taken intoa clean room environment (e.g., Class 1000 or better) for contactbonding. Single instances of the anodically-bonded chips were pairedwith single instances of the glass chips to define a pair of chips forcontact bonding. For each pair, a planar surface defined by the 100-nmlayer of SiO₂ on the silicon chip and a planar surface of the glass chipwere wiped with optical paper and acetone to clean any macroscopicdeposits or contaminants from them. Each pair was then submerged in anacetone bath (e.g., acetone in a beaker) and cleaned via ultrasoniccleaning for 15 minutes. Each pair of chips was subsequently removedfrom the acetone bath, rinsed with isopropanol (e.g., submerged in anisopropanol bath), and blown dry with dry nitrogen gas.

A pair of chips was placed in a YES-CV200RFS plasma cleaner and cleanedfor 45 seconds using a nitrogen plasma. (In some instances, multiplepairs of chips were place in the plasma cleaner.) In particular, theplanar surface defined by the 100-nm layer of SiO₂ on the silicon chipand the planar surface of the glass chip were activated by plasmacleaning. The RF-power of the plasma cleaner was set at about 75 W, andthe pressure inside was maintained at about 150 mTorr. Nitrogen gasintroduced into the plasma cleaner at a volume flow rate of about 20sccm. After activation by plasma cleaning, the pair of chips was removedfrom the YES-CV200RFS plasma cleaner and rinsed in de-ionized water for5 minutes. The rinsing process served to hydroxylate the activatedsurfaces. In some variations, the rinsing process was conducted with abasic aqueous solution (e.g., an aqueous solution of ammoniumhydroxide). Care was taken not to touch the two hydroxylated andactivated surfaces together.

The pair of chips was then transferred into a vacuum chamber and mountedinto a fixture having a “press finger”. The fixture held the glass chipadjacent the silicon chip of the anodically-bonded chip to define a gap.The activated and hydroxylated surface of the glass chip faced theactivated and hydroxylated SiO₂ surface of the silicon chip. The vacuumchamber was then sealed and pumped down to a reduced pressure (e.g.,less than 10⁻³ Torr) to remove volatile species (e.g., water vapor) thatmight react with a vapor of cesium atoms used to fill the cavities ofthe anodically-bonded chip. The fixture was then chilled to by athermoelectric cooler, which in turn, chilled at least theanodically-bonded chip to a temperature between −20° C. and 0° C.

After the temperatures of the pair of chips stabilized, the vapor ofcesium atoms was introduced into the vacuum chamber by opening a valveconnecting a source of cesium vapor to the vacuum chamber. The source ofthe cesium vapor was an oven containing a mass of cesium heated to aprocessing temperature. A target pressure of cesium vapor in the vacuumchamber could be controlled by altering an opening of the valve,altering the processing temperature induced by the oven, or both. Oncethe pressure in the vacuum chamber stabilized to the target pressure ofcesium vapor, the pair of chips was exposed to the vapor of cesium atomsfor a length of time.

The pressure of cesium vapor in the vacuum chamber influences the lengthof time needed to fill the anodically-bonded chip. One or both of thepressure of cesium vapor in the vacuum chamber and the period of timecan be varied to control an amount of cesium vapor that condenses in thecavities of the anodically-bonded chip. Once the length of time hadelapsed, the value to the source of cesium vapor was closed. The vacuumchamber was subsequently pumped down to the reduced pressure (e.g., lessthan 10⁻³ Torr) and the power to the thermoelectric cooler turned off.

Once the pair of chips reached ambient temperature, the fixture wasactuated to contact the activated and hydroxylated surface of the glasschip to the activated and hydroxylated SiO₂ surface of the silicon chip.The “press finger” was used to hold the contacted surfaces together for20 minutes, which drove the formation of a contact bond. In somevariations, the “press finger” was used to apply a target pressure(e.g., about 2 MPa) during the 20-minute duration.

Example 2

A thick glass wafer was obtained from Howard Glass Co., Inc. with athickness of 1 mm and a diameter of 4 inches. The thick glass wafer hada surface roughness, R_(a), no greater than 1 nm on each side.Electrical properties of the silicon wafer included a resistance thatranged from 0.1 Ω-cm to 0.3 Ω-cm. A thin glass wafer formed ofborosilicate glass was also obtained from Schott. The thin glass waferwas a MEMpax wafer having a diameter of 4 inches and a thickness of 300μm. The surface roughness was less than 0.5 nm. The thick and thin glasswafers were inspected in preparation for anodic and contact bonding. Inparticular, the glass wafers were visually inspected for chips,micro-cracks, and scratches. The wafers were also verified to have asurface roughness less than 1 nm.

Next, multiple thick glass chips were cut from the thick glass waferusing either a Protolaser R micro-laser tool or a DISCO DAD 3240 dicingsaw. Each thick glass chip had dimensions of 10 mm×20 mm. Nine holeswere subsequently machined through each of the thick glass chips with aProtolaser R micro-laser tool. The holes were each circular with a 1-mmdiameter or square with a 1-mm edge length. In some cases, combinationsof circles and holes were machined in a thick glass chip. The thickglass chips were inspected visually with 5× and 10× magnification loupesfor cracks or chips that might have occurred during cutting. Thick glasschips with zero or minimal surface defects were selected for subsequentvapor-cell fabrication.

The selected thick glass chips were then cleaned with acetone andisopropanol using cotton swabs and optical tissue paper. An ultrasoniccleaner was optionally used to assist the cleaning process by agitatingbaths of acetone or isopropanol in which the selected thick glass chipswere submerged. A less than 1 μm layer of Si was then grown on one sideof the thick glass chips using plasma-enhanced chemical vapor deposition(PECVD). A thickness uniformity of the Si layer was verified to bewithin ±3 nm over an area of a thick glass chip. Thick glass chipsfailing the uniformity criterion were discarded.

The thick glass chips were then cleaned with methanol and isopropanolusing cotton swabs and optical tissue paper to eliminate loose residueson their surfaces (e.g., such as due to handling). The thick glass chipswere subsequently deep-cleaned with acetone and isopropanol using cottonswabs and optical tissue paper. A low magnification loupe (e.g., 10×)was used during the deep cleaning process for a first visual inspectionfollowed by a high magnification microscope (e.g., 50×-200×) for asecond visual inspection. Thick glass chips passing the second visioninspection were placed in a bath of acetone for ultrasonic cleaning at40 kHz (e.g., in a Branson Ultrasonic Cleaner CPX-952-117R). Forexample, the thick glass chips could be placed in a glass beaker ofacetone and ultrasonically cleaned for 20 minutes at room temperature.After ultrasonic cleaning, the thick glass chips were dried withparticulate-free compressed air and stored in an air-tight containeruntil needed for bonding.

Separately, a dicing saw was used to cut the thin glass wafers intosuitable sizes for bonding to the (stored) thick glass chips. Two thinglass chips were prepared for each thick glass chip. If a thin glasschip was intended for an anodic bond, the thin glass chip was cut tohave the same dimensions as the thick glass chip. However, if a thinglass chip was intended for a contact bond, the thin glass chip was cutto have longer dimensions than the thick glass chip. For example, thinglass chips for anodic bonding had dimensions of 10 mm×20 mm and thinglass chips for contact bonding had dimensions of 10 mm×35 mm. Aftercutting, each thin glass chip was inspected to ensure that its opticalclarity was not degraded (e.g., hazing), or that scratches or crackswere not present. Then glass chips found to be acceptable were thencleaned with acetone using cotton swabs and optical tissue paper. Ifnecessary, the thin glass chips were placed in a glass beaker of acetoneand ultrasonically cleaned form 20 minutes at room temperature. Afterultrasonic cleaning, the glass chips were dried with particulate-freecompressed air and then stored in an air-tight container until neededfor bonding.

One thick glass chip (with a layer of Si up to 1 μm thick) and one thinglass chip were then placed into an assembly for anodic bonding. For thethick glass chip, the planar surface defined by the up to 1 μm layer ofSi participated in the anodic bonding process. In the assembly, planarsurfaces of the thick and thin glass chips were contacted to define aninterface, and the interface was visually inspected to confirm thatoptical fringes were present. The thick glass chip was then heated to atemperature of about 400° C. After this temperature was reached, 600Vwas applied across the thick and thin glass chips for about 15 minutes,which drove the formation of an anodic bond. The interface was inspectedagain to confirm the disappearance of the optical fringes, whichindicated the anodic bond was complete. Next, the anodic bond wasinspected for defects (e.g., bubbles, micro-cracks, unbonded areas,etc.). If 80% or more of an area around the holes was free of defects,the anodic bond was then further inspected for open channels (e.g., froma hole to the environment, a hole to another hole, etc.). If an openchannel was discovered, the anodically-bonded chips were discarded asthe anodic bond was not deemed leak-tight.

Bonded thick and thin glass chips with leak-tight anodic bonds werecleaned in acetone and methanol. During this cleaning process, theunbonded surface of the thick glass chip was cleaned with acetone andmethanol using cotton swabs and optical tissue paper to eliminate anyresidues (e.g., residues from a graphite plate of the assembly used toform the anodic bond). The unbonded surface of the thick glass chip wasthen visually inspected to ensure defects (e.g., scratches, pitting,etc.) were not present that might compromise a soon-to-be formed contactbond. The anodically-bonded chips were then individually cleaned. Inparticular, the anodically-bonded chips were placed individually (i.e.,with no other chips) in a glass beaker of acetone and cleanedultrasonically for 20 minutes at room temperature. After ultrasoniccleaning, the anodically-bonded chips were dried with particulate-freecompressed air. A low magnification loupe (e.g., 10×) was used for afirst visual inspection of the anodically-bonded chips, followed by ahigh magnification microscope (e.g., 50×-200×) for a second visualinspection. The first and second visual inspections were used to ensureno visual residues or deposits remained on the anodically-bonded chips.

The anodically-bonded chips—along with unbonded thin glass chips—werethen taken into a clean room environment (e.g., Class 1000 or better)for contact bonding. Single instances of anodically-bonded chips werepaired with single instances of thin glass chips to define a pair forcontact bonding. For each pair, an unbonded planar surface of the thickglass chip (i.e., without the layer of Si up to 1 μm) and a planarsurface of the thin glass chip were wiped with optical paper and acetoneto clean any macroscopic deposits or contaminants from them. Each pairwas then submerged in an acetone bath (e.g., acetone in a beaker) andcleaned via ultrasonic cleaning for 15 minutes. Each pair of chips wassubsequently removed from the acetone bath, rinsed with isopropanol(e.g., submerged in an isopropanol bath), and blown dry with drynitrogen gas.

A pair of chips was placed in a YES-CV200RFS plasma cleaner and cleanedfor 45 seconds using a nitrogen plasma. (In some instances, multiplepairs of chips were place in the plasma cleaner.) In particular, theunbonded planar surface of the thick glass chip and the planar surfaceof the glass chip were activated by plasma cleaning. The RF-power of theplasma cleaner was set at about 75 W, and the pressure inside wasmaintained at about 150 mTorr. Nitrogen gas introduced into the plasmacleaner at a volume flow rate of about 20 sccm. After activation byplasma cleaning, the pair of chips was removed from the YES-CV200RFSplasma cleaner and rinsed in de-ionized water for 5 minutes. The rinsingprocess served to hydroxylate the activated surfaces. In somevariations, the rinsing process was conducted with a basic aqueoussolution (e.g., an aqueous solution of ammonium hydroxide). Care wastaken not to touch the two hydroxylated and activated surfaces together.

The pair of chips was then transferred into a vacuum chamber and mountedinto a fixture having a “press finger”. The fixture held the thin glasschip adjacent the thick glass chip of the anodically-bonded chip todefine a gap. The activated and hydroxylated surface of the thin glasschip faced the activated and hydroxylated unbonded surface of the thickglass chip. The vacuum chamber was then sealed and pumped down to areduced pressure (e.g., less than 10⁻³ Torr) to remove volatile species(e.g., water vapor) that might react with a vapor of cesium atoms usedto fill the cavities of the anodically-bonded chip. The fixture was thenchilled to by a thermoelectric cooler, which in turn, chilled at leastthe anodically-bonded chip to a temperature between −20° C. and 0° C.

After the temperatures of the pair of chips stabilized, the vapor ofcesium atoms was introduced into the vacuum chamber by opening a valveconnecting a source of cesium vapor to the vacuum chamber. The source ofthe cesium vapor was an oven containing a mass of cesium heated to aprocessing temperature. A target pressure of cesium vapor in the vacuumchamber could be controlled by altering an opening of the valve,altering the processing temperature induced by the oven, or both. Oncethe pressure in the vacuum chamber stabilized to the target pressure ofcesium vapor, the pair of chips was exposed to the vapor of cesium atomsfor a length of time.

The pressure of cesium vapor in the vacuum chamber influences the lengthof time needed to fill the anodically-bonded chip. One or both of thepressure of cesium vapor in the vacuum chamber and the length of timecan be varied to control an amount of cesium vapor that condenses in thecavities of the anodically-bonded chip. Once the length of time hadelapsed, the value to the source of cesium vapor was closed. The vacuumchamber was subsequently pumped down to the reduced pressure (e.g., lessthan 10⁻³ Torr) and the power to the thermoelectric cooler turned off.

Once the pair of chips reached ambient temperature, the fixture wasactuated to contact the activated and hydroxylated surface of the glasschip to the activated and hydroxylated unbonded surface of the thickglass chip. The “press finger” was used to hold the contacted surfacestogether for 20 minutes, which drove the formation of a contact bond. Insome variations, the “press finger” was used to apply a target pressure(e.g., about 2 MPa) during the 20-minute duration.

In some aspects of what is described, a method of manufacturing a vaporcell may be also be described by the following examples:

Example 1

A method of manufacturing a vapor cell, the method comprising:

-   -   obtaining a dielectric body comprising a surface that defines an        opening to a cavity in the dielectric body;    -   obtaining an optical window that comprises a surface;    -   altering the surface of the dielectric body and the surface of        the optical window to comprise, respectively, a first plurality        of hydroxyl ligands and a second plurality of hydroxyl ligands;    -   disposing a vapor or a source of the vapor into the cavity; and    -   contacting the altered surface of the dielectric body to the        altered surface of the optical window to form a seal around the        opening to the cavity, the seal comprising metal-oxygen bonds        formed by reacting the first plurality of hydroxyl ligands with        the second plurality of hydroxyl ligands during contact of the        altered surfaces.

Example 2

The method of example 1, wherein contacting the altered surfacescomprises covering the opening of the cavity with the optical window toenclose the vapor or the source of the vapor in the cavity.

Example 3

The method of example 1 or example 2, wherein the dielectric body isformed of silicon.

Example 4

The method of example 3, comprising:

-   -   forming an adhesion layer on the dielectric body that defines        the surface of the dielectric body, the adhesion layer        comprising silicon oxide.

Example 5

The method of example 1 or example 2, wherein the dielectric body isformed of a glass comprising silicon oxide.

Example 6

The method of example 1 or any one of examples 2-5, wherein themetal-oxygen bonds comprise siloxane bonds.

Example 7

The method of example 1 or any one of examples 2-6, wherein the opticalwindow comprises silicon oxide.

Example 8

The method of example 1 or any one of examples 2-7, wherein the vaporcomprises a gas of alkali-metal atoms.

Example 9

The method of example 1 or any one of examples 2-7, wherein the vaporcomprises a gas of diatomic halogen molecules.

Example 10

The method of example 1 or any one of examples 2-7, wherein the vaporcomprises a gas of organic molecules.

Example 11

The method of example 1 or any one of examples 2-8, wherein disposingthe vapor or the source of the vapor comprises exposing the cavity to avacuum environment comprising a gas of alkali-metal atoms.

Example 12

The method of example 1 or any one of examples 2-8,

-   -   wherein disposing the vapor or the source of the vapor comprises        disposing a solid or liquid source of alkali-metal atoms into        the cavity; and    -   wherein the method comprises:        -   after contacting, heating the solid or liquid source of            alkali-metal atoms to generate a gas of the alkali-metal            atoms.

Example 13

The method of example 1 or any one of examples 2-12, wherein the vaporcomprises a noble gas.

Example 14

The method of example 1 or any one of examples 2-13, wherein alteringthe surfaces comprises activating one or both of the surfaces of thedielectric body and the optical window by exposing the respectivesurfaces to a plasma.

Example 15

The method of example 14, wherein altering the surfaces furthercomprises washing one or both of the activated surfaces of thedielectric body and the optical window in a basic aqueous solution.

Example 16

The method of example 1 or any one of examples 2-15, wherein the surfaceof the dielectric body and the surface of the optical window are planarsurfaces.

Example 17

The method of example 1 or any one of examples 2-16, wherein the surfaceof the dielectric body and the surface of the optical window have asurface roughness, R_(a), no greater than 1 nm.

Example 18

The method of example 1 or any one of examples 2-17, wherein contactingthe altered surfaces comprises pressing the altered surfaces of thedielectric body and the optical window against each other.

Example 19

The method of example 1 or any one of examples 2-18, comprising:

-   -   heating one or both the dielectric body and the optical window        after contacting the altered surfaces; and    -   while heating, clamping the dielectric body and the optical        window together to hold the altered surfaces in contact.

Example 20

The method of example 1 or any one of examples 2-19, wherein obtainingthe dielectric body comprises removing material from the dielectric bodyto form the cavity.

Example 21

The method of example 20, wherein removing material comprises machiningmaterial from the surface of the dielectric body with a laser.

Example 22

The method of example 20 or example 21, wherein removing materialcomprises etching material from the surface of the dielectric body.

In some aspects of what is described, a vapor cell may be also bedescribed by the following examples:

Example 23

A vapor cell, comprising:

-   -   a dielectric body comprising a surface that defines an opening        to a cavity in the dielectric body;    -   a vapor or a source of the vapor in the cavity of the dielectric        body; and    -   an optical window covering the opening of the cavity and having        a surface bonded to the surface of the dielectric body to form a        seal around the opening, the seal comprising metal-oxygen bonds        formed by reacting a first plurality of hydroxyl ligands on the        surface of the dielectric body with a second plurality of        hydroxyl ligands on the surface of the optical window.

Example 24

The vapor cell of example 23, wherein the dielectric body is formed ofsilicon.

Example 25

The vapor cell of example 24, wherein the vapor cell comprises anadhesion layer on the dielectric body that defines the surface of thedielectric body, the adhesion layer comprising silicon oxide.

Example 26

The vapor cell of example 23, wherein the dielectric body is formed of aglass comprising silicon oxide.

Example 27

The vapor cell of example 23 or any one of examples 24-26, wherein themetal-oxygen bonds comprise siloxane bonds.

Example 28

The vapor cell of example 23 or any one of examples 24-27, wherein theoptical window comprises silicon oxide.

Example 29

The vapor cell of example 23 or any one of examples 24-28, wherein thevapor comprises a gas of alkali-metal atoms.

Example 30

The vapor cell of example 23 or any one of examples 24-28, wherein thevapor comprises a gas of diatomic halogen molecules.

Example 31

The vapor cell of example 23 or any one of examples 24-28, wherein thevapor comprises a gas of organic molecules.

Example 32

The vapor cell of example 23 or any one of examples 24-29,

-   -   wherein the source of the vapor resides in the cavity of the        dielectric body; and    -   wherein the source of the vapor comprises a liquid or solid        source of alkali-metal atoms configured to generate a gas of the        alkali-metal atoms when heated.

Example 33

The vapor cell of example 23 or any one of examples 24-32, wherein thevapor comprises a noble gas.

Example 34

The vapor cell of example 23 or any one of examples 24-33, wherein thesurface of the dielectric body and the surface of the optical windowhave a surface roughness, R_(a), no greater than 1 nm.

In some aspects of what is described, a method of manufacturing a vaporcell that has at least two optical windows may be also be described bythe following examples:

Example 35

A method of manufacturing a vapor cell that has at least two opticalwindows, the method comprising:

-   -   obtaining a dielectric body comprising:        -   a cavity in the dielectric body,        -   a first surface that defines a first opening to the cavity,            and        -   a second surface that defines a second opening to the            cavity;    -   obtaining a first optical window that comprises a surface;    -   bonding the surface of the first optical window to the first        surface of the dielectric body to form a first seal around the        first opening to the cavity;    -   obtaining a second optical window that comprises a surface;    -   altering the second surface of the dielectric body and the        surface of the second optical window to comprise, respectively,        a first plurality of hydroxyl ligands and a second plurality of        hydroxyl ligands;    -   disposing a vapor or a source of the vapor into the cavity        through the second opening;    -   contacting the altered second surface of the dielectric body to        the altered surface of the second optical window to form a        second seal around the second opening to the cavity, the second        seal comprising metal-oxygen bonds formed by reacting the first        plurality of hydroxyl ligands with the second plurality of        hydroxyl ligands during contact of the altered surfaces.

Example 36

The method of example 35, wherein bonding the surface of the firstoptical window to the first surface of the dielectric body includescovering the first opening of the cavity with the first optical window.

Example 37

The method of example 35 or example 36, wherein contacting the alteredsurfaces comprises covering the second opening of the cavity with thesecond optical window to enclose the vapor or the source of the vapor inthe cavity.

Example 38

The method of example 35 or any one of examples 36-37, wherein thedielectric body is formed of silicon.

Example 39

The method of example 38, comprising:

-   -   forming an adhesion layer on the dielectric body that defines        the second surface of the dielectric body, the adhesion layer        comprising silicon oxide.

Example 40

The method of example 35 or any one of examples 36-37, wherein thedielectric body is formed of a glass comprising silicon oxide.

Example 41

The method of example 35 or any one of examples 36-40, wherein themetal-oxygen bonds of the second seal comprise siloxane bonds.

Example 42

The method of example 35 or any one of examples 36-41, wherein the firstand second optical windows comprise silicon oxide.

Example 43

The method of example 35 or any one of examples 36-42, wherein the vaporcomprises a gas of alkali-metal atoms.

Example 44

The method of example 35 or any one of examples 36-42, wherein the vaporcomprises a gas of diatomic halogen molecules.

Example 45

The method of example 35 or any one of examples 36-42, wherein the vaporcomprises a gas of organic molecules.

Example 46

The method of example 35 or any one of examples 36-43, wherein disposingthe vapor or the source of the vapor comprises exposing the cavity to avacuum environment comprising a gas of alkali-metal atoms.

Example 47

The method of example 35 or any one of examples 36-43,

-   -   wherein disposing the vapor or the source of the vapor comprises        disposing a solid or liquid source of alkali-metal atoms into        the cavity through the second opening; and    -   wherein the method comprises:        -   after contacting, heating the solid or liquid source of            alkali-metal atoms to generate a gas of the alkali-metal            atoms.

Example 48

The method of example 35 or any one of examples 36-47, wherein the vaporcomprises a noble gas.

Example 49

The method of example 35 or any one of examples 36-48, wherein alteringthe surfaces comprises activating one or both of the second surface ofthe dielectric body and the surface of the second optical window byexposing the respective surfaces to a plasma.

Example 50

The method of example 49, wherein altering the surfaces furthercomprises washing one or both of the activated surfaces of thedielectric body and the second optical window in a basic aqueoussolution.

Example 51

The method of example 35 or any one of examples 36-50, wherein thesecond surface of the dielectric body and the surface of the secondoptical window have a surface roughness, R_(a), no greater than 1 nm.

Example 52

The method of example 35 or any one of examples 36-51, whereincontacting the altered surfaces comprises pressing the altered secondsurface of the dielectric body and the altered surface of the secondoptical window against each other.

Example 53

The method of example 35 or any one of examples 36-52, comprising:

-   -   heating one or both the dielectric body and the second optical        window after contacting the altered surfaces; and    -   while heating, clamping the dielectric body and the second        optical window together to hold the altered surfaces in contact.

Example 54

The method of example 35 or any one of examples 36-53, wherein bondingthe surface of the first optical window comprises:

-   -   altering the first surface of the dielectric body and the        surface of the first optical window to comprise, respectively, a        third plurality of hydroxyl ligands and a fourth plurality of        hydroxyl ligands; and    -   contacting the altered first surface of the dielectric body to        the altered surface of the first optical window to form the        first seal around the first opening to the cavity, the first        seal comprising metal-oxygen bonds formed by reacting the third        plurality of hydroxyl ligands with the fourth plurality of        hydroxyl ligands during contact of the altered surfaces.

Example 55

The method of example 35 or any one of examples 36-39 and 41-53(excluding the subject matter of example 40 in any combination ofexamples that includes example 55),

-   -   wherein the dielectric body is formed of silicon and the first        optical window comprises silicon oxide; and    -   wherein bonding the surface of the first optical window        comprises anodically bonding the surface of the first optical        window to the first surface of the dielectric body to form the        first seal.

Example 56

The method of example 35 or any one of examples 36-37 and 40-53(excluding the subject matter of examples 38-39 in any combination ofexamples that includes example 56),

-   -   wherein the dielectric body is formed of a glass comprising        silicon oxide and the first optical window comprises silicon        oxide;    -   wherein the method comprises depositing a layer of silicon on        the first surface of the dielectric body; and    -   wherein bonding the surface of the first optical window        comprises anodically bonding the layer of silicon to the surface        of the first optical window to form the first seal.

Example 57

The method of example 35 or any one of examples 36-37 and 40-53(excluding the subject matter of examples 38-39 in any combination ofexamples that includes example 57),

-   -   wherein the dielectric body is formed of a glass comprising        silicon oxide and the first optical window comprises silicon        oxide; and    -   wherein bonding the surface of the first optical window        comprises:        -   applying a glass frit to one or both of the first surface of            the dielectric body and the surface of the first optical            window,        -   contacting the first surface of the dielectric body to the            surface of the first optical window, and        -   heating at least one of the glass frit, the dielectric body,            or the first optical window to a firing temperature to form            the first seal.

Example 58

The method of example 35 or any one of examples 36-57,

-   -   wherein the first and second surfaces of the dielectric body are        planar surfaces opposite each other; and    -   wherein the surface of the first optical window and the surface        of second optical window are planar surfaces.

Example 59

The method of example 35 or any one of examples 36-58, wherein obtainingthe dielectric body comprises removing material from the dielectric bodyto form the cavity.

Example 60

The method of example 59, wherein removing material comprises machiningmaterial from the surface of the dielectric body with a laser.

Example 61

The method of example 59 or example 60, wherein removing materialcomprises etching material from the surface of the dielectric body.

In some aspects of what is described, a vapor cell having at least twooptical windows may be also be described by the following examples:

Example 62

A vapor cell having at least two optical windows, the vapor cellcomprising:

-   -   a dielectric body comprising:        -   a cavity in the dielectric body,        -   a first surface that defines a first opening to the cavity,            and        -   a second surface that defines a second opening to the            cavity;    -   a vapor or a source of the vapor in the cavity of the dielectric        body;    -   a first optical window covering the first opening of the cavity        and having a surface bonded to the first surface of the        dielectric body to form a first seal around the first opening;        and    -   a second optical window covering the second opening of the        cavity and having a surface bonded to the second surface of the        dielectric body to form a second seal around the second opening,        the second seal comprising metal-oxygen bonds formed by reacting        a first plurality of hydroxyl ligands on the second surface of        the dielectric body with a second plurality of hydroxyl ligands        on the surface of the second optical window.

Example 63

The vapor cell of example 62, wherein the dielectric body is formed ofsilicon.

Example 64

The vapor cell of example 63, wherein the vapor cell comprises anadhesion layer on the dielectric body that defines the second surface ofthe dielectric body, the adhesion layer comprising silicon oxide.

Example 65

The vapor cell of example 62, wherein the dielectric body is formed of aglass comprising silicon oxide.

Example 66

The vapor cell of example 62 or any one of examples 63-65, wherein themetal-oxygen bonds of the second seal comprise siloxane bonds.

Example 67

The vapor cell of example 62 or any one of examples 63-66, wherein thefirst and second optical windows comprise silicon oxide.

Example 68

The vapor cell of example 62 or any one of examples 63-67, wherein thevapor comprises a gas of alkali-metal atoms.

Example 69

The vapor cell of example 62 or any one of examples 63-67, wherein thevapor comprises a gas of diatomic halogen molecules.

Example 70

The vapor cell of example 62 or any one of examples 63-67, wherein thevapor comprises a gas of organic molecules.

Example 71

The vapor cell of example 62 or any one of examples 63-68,

-   -   wherein the source of the vapor resides in the cavity of the        dielectric body; and    -   wherein the source of the vapor comprises a liquid or solid        source of alkali-metal atoms configured to generate a gas of the        alkali-metal atoms when heated.

Example 72

The vapor cell of example 62 or any one of examples 63-71, wherein thevapor comprises a noble gas.

Example 73

The vapor cell of example 62 or any one of examples 63-72, wherein thesecond surface of the dielectric body and the surface of the secondoptical window have a surface roughness, R_(a), no greater than 1 nm.

Example 74

The vapor cell of example 62 or any one of examples 63-73,

-   -   wherein the first and second surfaces of the dielectric body are        planar surfaces opposite each other; and    -   wherein the surface of the first optical window and the surface        of second optical window are planar surfaces.

Example 75

The vapor cell of example 62 or any one of examples 63-74, wherein thefirst seal comprises metal-oxygen bonds formed by reacting a thirdplurality of hydroxyl ligands on the first surface of the dielectricbody with a fourth plurality of hydroxyl ligands on the surface of thefirst optical window.

Example 76

The vapor cell of example 62 or any one of examples 63-74, wherein thedielectric body is formed of silicon and the first optical windowcomprises silicon oxide.

Example 77

The vapor cell of example 76, wherein the first seal comprises an anodicbond between the first surface of the dielectric body and the surface ofthe first optical window.

Example 78

The vapor cell of example 62 or any one of examples 63-74,

-   -   wherein the dielectric body is formed of a glass comprising        silicon oxide and the first optical window comprises silicon        oxide;    -   wherein the vapor cell comprises a layer of silicon disposed        between the first surface of the dielectric body and the surface        of the first optical window; and    -   wherein the first seal comprises an anodic bond between the        layer of silicon and one or both of the first surface of the        dielectric body and the surface of the first optical window.

Example 79

The vapor cell of example 62 or any one of examples 63-74,

-   -   wherein the dielectric body is formed of a glass comprising        silicon oxide and the first optical window comprises silicon        oxide; and    -   wherein the vapor cell comprises a fired layer of glass frit        bonding the first surface of the dielectric body to the surface        of the first optical window, the fired layer of glass frit        defining the first seal.

While this specification contains many details, these should not beunderstood as limitations on the scope of what may be claimed, butrather as descriptions of features specific to particular examples.Certain features that are described in this specification or shown inthe drawings in the context of separate implementations can also becombined. Conversely, various features that are described or shown inthe context of a single implementation can also be implemented inmultiple embodiments separately or in any suitable sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single product or packagedinto multiple products.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications can be made. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A vapor cell, comprising: a dielectric bodycomprising a surface that defines an opening to a cavity in thedielectric body; an adhesion layer on the surface of the dielectric bodyand surrounding the opening of the cavity, the adhesion layer having anexterior surface opposite the surface of the dielectric body andcomprising silicon oxide; a vapor or a source of the vapor in the cavityof the dielectric body; and an optical window covering the opening ofthe cavity and having a surface bonded to the exterior surface of theadhesion layer to form a seal around the opening, the seal comprisingmetal-oxygen bonds formed by reacting a first plurality of hydroxylligands on the exterior surface of the adhesion layer with a secondplurality of hydroxyl ligands on the surface of the optical window. 2.The vapor cell of claim 1, wherein the dielectric body is formed ofsilicon.
 3. The vapor cell of claim 1, wherein the dielectric body isformed of a glass comprising silicon oxide.
 4. The vapor cell of claim1, wherein the metal-oxygen bonds comprise siloxane bonds.
 5. The vaporcell of claim 1, wherein the optical window comprises silicon oxide. 6.The vapor cell of claim 1, wherein the vapor comprises a gas ofalkali-metal atoms.
 7. The vapor cell of claim 1, wherein the vaporcomprises a noble gas.
 8. The vapor cell of claim 1, wherein the vaporcomprises a gas of diatomic halogen molecules.
 9. The vapor cell ofclaim 1, wherein the vapor comprises a gas of organic molecules.
 10. Thevapor cell of claim 1, wherein the source of the vapor resides in thecavity of the dielectric body; and wherein the source of the vaporcomprises a liquid or solid source of alkali-metal atoms configured togenerate a gas of the alkali-metal atoms when heated.
 11. The vapor cellof claim 1, wherein the surface of the dielectric body and the surfaceof the optical window have a surface roughness, R_(a), no greater than 1nm.
 12. A vapor cell having at least two optical windows, the vapor cellcomprising: a dielectric body comprising: a cavity in the dielectricbody, a first surface that defines a first opening to the cavity, and asecond surface that defines a second opening to the cavity; an adhesionlayer on the second surface of the dielectric body and surrounding thesecond opening of the cavity, the adhesion layer having an exteriorsurface opposite the second surface of the dielectric body andcomprising silicon oxide; a vapor or a source of the vapor in the cavityof the dielectric body; a first optical window covering the firstopening of the cavity and having a surface bonded to the first surfaceof the dielectric body to form a first seal around the first opening;and a second optical window covering the second opening of the cavityand having a surface bonded to the exterior surface of the adhesionlayer to form a second seal around the second opening, the second sealcomprising metal-oxygen bonds formed by reacting a first plurality ofhydroxyl ligands on the exterior surface of the adhesion layer with asecond plurality of hydroxyl ligands on the surface of the secondoptical window.
 13. The vapor cell of claim 12, wherein the dielectricbody is formed of silicon.
 14. The vapor cell of claim 12, wherein thedielectric body is formed of a glass comprising silicon oxide.
 15. Thevapor cell of claim 12, wherein the metal-oxygen bonds of the secondseal comprise siloxane bonds.
 16. The vapor cell of claim 12, whereinthe first and second optical windows comprise silicon oxide.
 17. Thevapor cell of claim 12, wherein the vapor comprises a gas ofalkali-metal atoms.
 18. The vapor cell of claim 12, wherein the vaporcomprises a noble gas.
 19. The vapor cell of claim 12, wherein the vaporcomprises a gas of diatomic halogen molecules.
 20. The vapor cell ofclaim 12, wherein the vapor comprises a gas of organic molecules. 21.The vapor cell of claim 12, wherein the source of the vapor resides inthe cavity of the dielectric body; and wherein the source of the vaporcomprises a liquid or solid source of alkali-metal atoms configured togenerate a gas of the alkali-metal atoms when heated.
 22. The vapor cellof claim 12, wherein the second surface of the dielectric body and thesurface of the second optical window have a surface roughness, R_(a), nogreater than 1 nm.
 23. The vapor cell of claim 12, wherein the first andsecond surfaces of the dielectric body are planar surfaces opposite eachother; and wherein the surface of the first optical window and thesurface of second optical window are planar surfaces.
 24. The vapor cellof claim 12, wherein the first seal comprises metal-oxygen bonds formedby reacting a third plurality of hydroxyl ligands on the first surfaceof the dielectric body with a fourth plurality of hydroxyl ligands onthe surface of the first optical window.
 25. The vapor cell of claim 12,wherein the dielectric body is formed of silicon and the first opticalwindow comprises silicon oxide.
 26. The vapor cell of claim 25, whereinthe first seal comprises an anodic bond between the first surface of thedielectric body and the surface of the first optical window.
 27. Thevapor cell of claim 12, wherein the dielectric body is formed of a glasscomprising silicon oxide and the first optical window comprises siliconoxide; wherein the vapor cell comprises a layer of silicon disposedbetween the first surface of the dielectric body and the surface of thefirst optical window; and wherein the first seal comprises an anodicbond between the layer of silicon and one or both of the first surfaceof the dielectric body and the surface of the first optical window. 28.The vapor cell of claim 12, wherein the dielectric body is formed of aglass comprising silicon oxide and the first optical window comprisessilicon oxide; and wherein the vapor cell comprises a fired layer ofglass frit bonding the first surface of the dielectric body to thesurface of the first optical window, the fired layer of glass fritdefining the first seal.